This invention relates to infrared radiation detectors, particularly, but not exclusively, to so-called multicolor detectors. Multicolor detectors measure different wavelengths emitted by a broadband source of infrared radiation. Such multicolor detectors permit, for example, measurement of the temperature of the radiation source independent of the emissivity of the source and the transmission of the space between the source and the detector. The detector may comprise either single detector elements or arrays of detector elements. The detector elements may be photoconductive or photovoltaic.
The invention further relates to a method of manufacturing such a detector.
In an article entitled "N-Color (Hg,CD)Te Photodetectors" by H. Halpert, et al (Applied Optics, Vol. 11, No. 10, October 1972, pages 2157 to 2161), infrared radiation detectors are described. Each detector comprises at least one first detector element and at least one second detector element. The first detector element is formed in a lower body of infrared sensitive material mounted on a substrate. The second detector element, which has different detector characteristics from those of the first detector element, is formed in an upper body of infrared sensitive material mounted on the lower body. Electrical connections are made to the detector elements of the upper and lower bodies. One of the first and second detector elements is positioned to receive infrared radiation transmitted by the other of the detector elements.
As illustrated in FIGS. 3 to 5 of the Halpert, et al article, the material of the lower body has a longer cutoff wavelength than that of the upper body. The lower first detector element therefore senses long wavelength radiation transmitted by the upper body. The electrical connections in these known detectors are made by bonding wires to indium contact metallizations actually on the detector element bodies themselves. Particularly with detector elements of cadmium mercury telluride, the bonding of these wire connections to metallizations on the infrared sensitive material can result in strain and damage to the infrared sensitive material. Such strain and damage degrade the performance of the detector elements by, for example, increasing charge carrier recombination. In extreme cases fracturing of the infrared sensitive material may even occur. It is also inconvenient and difficult to carefully bond these wire connections at the different levels of the upper and lower body surfaces.
Similar connection problems can arise not only in multicolor detectors but in any other type of infrared detector having stacked detector element bodies. One such other type of detector is described in U.S. Pat. No. 3,987,298 (Rotolante). In this patent, the first and second detector elements have the same cutoff wavelength, but the upper detector element body is thinner and transmits some proportion of the radiation to the lower detector element body. In the embodiment shown in FIG. 2 of U.S. Pat. No. 3,987,298, the first detector element has metallizations which extend onto the substrate where the wire connections are made. The upper body is separated from the lower body by a layer of transparent insulator such as epoxy which also covers part of the metallization on the substrate. The detector element of the upper body has metallizations which extend onto the insulator layer where the wire connections are made to the second detector element.
However this structure places conflicting requirements on the thickness of the intermediate insulator layer. On the one hand, this insulator layer generally should be very thin between the upper and lower bodies so as to reduce strain in the detector element bodies and to increase thermal conductance and infrared radiation transmission between the bodies. On the other hand, a very thin insulator layer between the metallization of the first and second detector elements may result in short-circuiting of the two levels of metallization as a result of pin holes in the insulator or damage to the insulator when bonding wires to the upper level metallization.
In the structure illustrated in FIG. 2 of U.S. Pat. No. 3,987,298, the insulator layer shown (as indicated by reference numeral 26) combines both undesirable features discussed above. The layer is undesirably thicker between the bodies (reference numerals A and B) and thinner between the two levels of metallization (reference numerals 22, 28, 24, and 30). Furthermore, this structure does not eliminate the problem of bonding wires at two different levels.
U.S. Pat. No. 4,206,470 (White) discloses a modified insulated electrical connection structure for multicolor, stacked infrared detector element array bodies. The detector array is mounted on a signal processing silicon CCD substrate in a focal plane imaging arrangement. In this detector arrangement, as illustrated in FIG. 2 of U.S. Pat. No. 4,206,470, gold or nickel first-level contact pads (such as reference numerals 134a, 134b and 134c) are plated up to extend upward from the substrate to at least the height of the top surface of the lower detector element body. The gaps between the body and the individual contact pads are back filled with an inert insulating material such as epoxy. The epoxy must then be mechanically lapped down to form exposed contact pads at an essentially coplanar surface with the remaining epoxy and with the top surface of the lower detector element body. Then, thin film metal interconnects are deposited on the coplanar surface to form the connections for the lower array of first detector elements.
The upper detector element body is then mounted on the lower body, and the whole sequence of plating contact pads, back filling with epoxy, mechanically lapping, and depositing thin film metal interconnects is repeated to form the connections for the upper array of second detector elements.
Such a manner of providing the connections for second detector elements is complex and involves many different processing steps. The processing steps are labor-intensive and can reduce the yield of detectors manufactured with satisfactory performance. Thus, for example, the very thick back filling with epoxy and the mechanical lapping can strain and damage the infrared sensitive material, increasing charge carrier recombination. The problems are particularly acute with a material such as cadmium mercury telluride.